Generally, for molding of resin-molded articles, a molding method in which thermoplastic resin in molten state is sandwiched between split mold blocks and molded has been widely used.
In such molding method, the resin material is brought into a molten state and then clamped with split mold blocks. The molds are then opened, releasing a molded article. Subsequently, the molded article is subjected to removal of burrs from the exterior or the like of the molded article with a cutter or the like to give a finished article.
Such a general molding method causes an undesired resin portion other than the finished article itself, which is the so-called burr or the like, on the exterior or the like of the finished article with respect to a resin material that has solidified by cooling after molding.
The resin portion other than the finished article has the thermal history of having been heated during molding for producing molten state and then solidified by cooling. Thus, the physical properties of such resin that can be expressed numerically, such as melt tension (MT), melt flow rate (MFR), and Izod impact strength, are in many cases degraded compared with those of virgin resin without thermal history.
From the viewpoint of resource saving and cost reduction, the mass-production of a resin-molded article carries out a repetitive cycle of pulverizing a resin portion other than the finished article, which has been generated in molding, recovering it as a recovered resin material; mixing the recovered resin material with virgin resin to obtain base material resin, and making it in a molten state, followed by molding for producing a subsequent resin-molded article.
In many cases, therefore, the resin material, which is used in a production cycle for mass production of the resin-molded article, causes decreases in its numerically expressed physical properties, such as MT, MFR, and Izod impact strength, compared with the case of using only virgin resin without thermal history.
Hence, the selection of a resin for molding is generally performed such that a resin with high physical properties can be selected to have sufficient properties even in a state of being mixed with recovered resin material and having degraded physical properties compared with virgin resin.
If the numerically expressed physical properties, such as MT and MFR, are degraded and drop below certain lines, pinholes may be formed when, during molding, the resin material in molten state is greatly extended so as to conform to the irregularities of the mold surface.
Further, a change in fluid property or carbonization of the resin may appear, and may therefore lead to a problem of, for example, an uneven wall thickness or surface degradation of the resin due to drawdown in blow molding.
Further, in the case of foamed resin, the foamed cells may be defoamed during cell growth. Thus, if the foamed cells are defoamed in this way, the expansion ratio is lowered and hardy becomes high.
One of techniques provided by the present applicant includes, for the purpose of molding for producing a lightweight foam-molded article with excellent impact resistance using a mixture resin including a polypropylene-based resin for foaming and a hydrogenated styrene-based thermoplastic elastomer, and making the styrene content of the hydrogenated styrene-based thermoplastic elastomer 15 to 25 wt % (see, for example, Patent Literature 1).